KABUL, Afghanistan—President Hamid Karzai on Sunday defied the assembly he had convened to consider a critical security agreement with Washington, rejecting the gathering's request for rapidly sealing the deal and repeating demands for a lengthy delay.

In a combative speech just after the Loya Jirga's 3,000 delegates approved the security agreement, Mr. Karzai voiced a litany of complaints about the U.S. He also named three...